B. C. LeGeyt

Waveform-Sampling Electronics for a Whole-Body Time-of-Flight PET Scanner

Waveform sampling is an appealing technique for instruments requiring precision time and pulse-height measurements. Sampling each photomultiplier tube (PMT) waveform at oscilloscope-like rates of several gigasamples per second enables one to process PMT signals digitally, which in turn makes it straightforward to optimize timing resolution and amplitude (energy and position) resolution in response to calibration effects, pile-up effects, and other systematic sources of waveform variation. We describe a system design and preliminary implementation that neatly maps waveform-sampling technology onto the LaPET prototype whole-body time-of-flight PET scanner that serves as the platform for testing this new technology.

Design and Performance of a High Spatial Resolution, Time-of-Flight PET Detector

This paper describes the design and performance of a high spatial resolution positron emission tomography (PET) detector with time-of-flight capabilities. With an emphasis on high spatial resolution and sensitivity, we initially evaluated the performance of several 1.5 ×1.5 and 2.0 ×2.0 mm2 and 12-15 mm long LYSO crystals read out by several appropriately sized PMTs. Experiments to evaluate the impact of reflector on detector performance were performed and the final detector consisted of a 32 ×32 array of 1.5 ×1.5 ×15 mm3 LYSO crystals packed with a diffuse reflector and read out by a single Hamamatsu 64 channel multi-anode PMT. Such a design made it compact, modular and offered a cost-effective solution to obtaining excellent energy and timing resolution. To minimize the number of readout signals, a compact front-end readout electronics that summed anode signals along each of the orthogonal directions was also developed. Experimental evaluation of detector performance demonstrates clear discrimination of the crystals within the detector. An average energy resolution (FWHM) of 12.7 ±2.6% and average coincidence timing resolution (FWHM) of 348 ps was measured, demonstrating suitability for use in the development of a high spatial resolution time-of-flight scanner for dedicated breast PET imaging.

Waveform-sampling electronics for time-of-flight PET scanner

Waveform sampling (WFS) is an appealing technique for instruments requiring precision time and pulse-height measurements. Recent advances in switched-capacitor-array ASICs such as the Domino Ring Sampler (DRS4) have made WFS affordable for large systems. LAPET is a whole-body time-of-flight PET scanner using 38880 LaBr3(5% Ce) scintillator crystals of dimension 4 × 4 × 30 mm3, imaged by 432 Photonis XP20D0 PMTs, grouped into 24 identical detector modules. High light yield (61000 photons/MeV) and fast decay time (20 ns) make LaBr3 an excellent scintillator for TOF PET. Our group previously reported coincidence timing resolution 315–330 ps (fwhm) in benchtop measurements and 375 ps in full-system measurements using semi-custom electronics. This contribution reports on a complete redesign of the LAPET electronics, trigger, and data acquisition system. Our design uses 240 DRS4 chips to obtain oscilloscope-quality sampling of each PMT waveform at 2 GSPS. The 7 PMTs with which each crystals scintillation light is collected map cleanly into the 8 analog inputs of a DRS4 chip, facilitating a redundant and nearly deadtime-free (at clinical rates) trigger design, in spite of the ∼ 3 µs required for DRS4 readout. An FPGA-based trigger using analog pulse shaping and 100 MSPS sampling provides coarse energy and timing measurements used to detect coincident pairs and to select DRS4 chips for readout. Simulation studies show that oscilloscope-quality readout of each PMT signal will permit more flexible handling of detector calibrations, PMT waveform baseline offsets, and pulse pile-up effects. We thus expect the upgraded electronics to permit system-level performance that more closely approximates single-module benchtop results and to preserve that performance at clinical count rates. Our goals are both to explore the feasibility of WFS for a large scanner and to improve the overall performance of the LAPET research scanner. We present initial tests using prototype units of our redesigned electronics.

Acceptance tests and criteria of the ATLAS transition radiation tracker

The Transition Radiation Tracker (TRT) sits at the outermost part of the ATLAS Inner Detector, encasing the Pixel Detector and the Semi-Conductor Tracker (SCT). The TRT combines charged particle track reconstruction with electron identification capability. This is achieved by layers of xenon-filled straw tubes with periodic radiator foils or fibers providing TR photon emission. The design and choice of materials have been optimized to cope with the harsh operating conditions at the LHC, which are expected to lead to an accumulated radiation dose of 10 Mrad and a neutron fluence of up to 2middot1014 n/cm2 after ten years of operation. The TRT comprises a barrel containing 52 000 axial straws and two end-cap parts with 320 000 radial straws. The total of 420 000 electronic channels (two channels per barrel straw) allows continuous tracking with many projective measurements (more than 30 straw hits per track). The assembly of the barrel modules in the US has recently been completed, while the end-cap wheel construction in Russia has reached the 50% mark. After testing at the production sites and shipment to CERN, all modules and wheels undergo a series of quality and conformity measurements. These acceptance tests survey dimensions, wire tension, gas-tightness, high-voltage stability and gas-gain uniformity along each individual straw. This paper gives details on the acceptance criteria and measurement methods. An overview of the most important results obtained to-date is also given

Initial imaging results from a high spatial-resolution time-of-flight PET detector designed for dedicated breast imaging

This paper discusses initial imaging results from a high-resolution time-of-flight detector specifically developed for a limited-angle dedicated breast PET scanner. To maintain high spatial-resolution and sensitivity, the detector design consists of 32 × 32 array of 1.5 × 1.5 × 15 mm3 LYSO crystals coupled to a single Hamamatsu H8500 multi-anode photomultiplier tube with a modified high-voltage divider circuit. To minimize the number of readout channels, compact front-end electronics that summed anode-signals along each of the orthogonal directions was also developed. Experimental performance evaluation of a complete detector-module demonstrates excellent energy, timing resolution and clear discrimination of most crystals. An average energy resolution of about 12.7% FWHM and an average coincidence timing resolution of 348 ps for two such detectors was measured. The dedicated breast PET scanner comprises of two parallel detector heads, each 15 cm by 10 cm and comprised of two rows of three detector-modules. We also experimentally evaluated the imaging capability of the scanner design via an experimental benchtop-demonstrator consisting of two fully assembled detector-modules on opposing translational stages. Imaging experiments with a hot lesion phantom that had an 8-mm diameter lesion with 8:1 activity uptake ratio, successfully demonstrate the capability of the system in imaging small lesions in a uniform background.

Combined analog/digital approach to performance optimization for the LAPET whole-body TOF PET scanner

LAPET is a LaBr3-based whole-body time-of-flight PET scanner. We previously reported coincidence timing resolution 315-330 ps (fwhm) in benchtop measurements and 375 ps in full-system measurements. We are currently testing prototype units for a complete redesign of LAPETs electronics, aimed at further improving full-system timing performance and at preserving that performance at high count rates. We report on four facets of the new design. First, PMT-by-PMT high-voltage control at two points per dynode chain permits both gains and timing offsets to be equalized across the scanner. Second, analog pulse shaping reduces the duration of each PMT pulse from 75 ns to 35 ns, reducing pile-up effects. Third, custom circuit boards use the DRS4 waveform-sampling ASIC to provide oscilloscope-quality readout of each PMT signal, enabling digital processing of PMT waveforms. Finally, an FPGA-based trigger provides the coarse energy and timing measurements used to detect coincident pairs. Tests are underway of prototype High Voltage Control boards, Shaper/Analog Mezzanine cards, and the DRS4-based Module Readout Board; the Master Coincidence Unit design is in progress.

ROCSTAR: Data acquisition electronics for TOF PET

Readout electronics are being developed to instrument a high resolution, time-of-flight PET scanner, targeted for breast imaging. The electronics and system design is a modular one, allowing the electronics to be used in many different system geometries. The electronics has been optimized for the H8500 PMT but it could be used to instrument a wide range of other rectangular photosensor arrays. The design of the electronics was presented, along with the results of simulations which were used to determine a set of electronics specifications. The simulations provided valuable guidance in setting out the design of the board. In particular the simulations indicated the need for channel-by-channel DRS4 calibration (to reduce sample jitter) as well as having two DRS4 chips in parallel (to reduce dead-time). The simulations also guided our choice of pulse shaping for the anode/positioning portion of the circuit. Prototype results show that the design falls well within the specifications that we had established.